The present invention relates to a system of drying and heating process air to be circulated for drying hygroscopic thermoplastic material contained in a drying hopper before the material is introduced into a plastic processing machine. The invention is concerned more particularly with improved desiccant towers used in a drying system for effectively drying thermoplastics in areas of high ambient humidity and for considerably reducing the time required to effectively and completely regenerate or purge moisture from a saturated desiccant tower.
The drying of thermoplastics before entering plastic processing machines such as injection, and extrusion molding machines is highly critical. Thermoplastic materials are generally hygroscopic. Moisture in ambient air can harm the mechanical, electrical, and visual properties of the finished plastic product. Hence, there has long been the need for reliable and efficient dryer systems to dry thermoplastic material contained in a plastics storage device such as a drying hopper before the material is processed. Drying thermoplastic material is usually accomplished by first drying process air which subsequently circulates through and drives moisture from thermoplastics contained in a drying hopper.
A standard implementation of a dryer system used with plastic processing machines comprises a desiccant tower containing desiccant (adsorbent) for adsorbing moisture from ambient or process air while the tower is in a process cycle. Adsorbents used are molecular solids oppositely ionized relative to that of water molecules. The water molecules are thereby electrically attracted and absorbed into the molecular solid. Some well-known types of solid desiccants used in dryer systems include molecular sieves, silica gel, and activated alumina.
The reason for using such adsorbents is due to their high moisture-holding capacity defined by the equation: equilibrium H.sub.2 O capacity=lb. of adsorbed H.sub.2 O/100 lb. of adsorbent. At a typical thermoplastics drying temperature of 150.degree. to 300.degree. Fahrenheit (F.), some molecular sieves can adsorb as much as 20% of their dry weight in moisture, such as water.
The desiccant, however, eventually becomes saturated, thereby losing its effectiveness for drying process air. Consequently, the saturated desiccant must be taken "off-stream" in order to regenerate its moisture adsorbing capacity. A heater located at or near the base of the saturated tower super-heats air circulating upwardly through the tower. The super-heated air transfers its thermal energy to the saturated desiccant by means of thermal convection. Moisture evaporates and is driven off the hot desiccant, whereupon the hot circulating air carries the evaporated moisture away from the saturated tower to be vented.
Obviously, the time (usually hours) necessary to regenerate the saturated desiccant cannot be used for drying thermoplastic material. In response to the off-stream problem, several solutions have been developed to ensure a constant supply of dry process air to the drying hopper even when a saturated tower is under regeneration (in a regeneration cycle).
As one solution, Conair dryer models CD-100 through CD-2400 employ four desiccant towers. When a saturated desiccant tower needs to be regenerated, a carrousel indexes a fresh desiccant tower to replace the saturated tower. Hence, desiccant tower indexing assures an uninterrupted supply of dry process air for drying thermoplastics. However, the drying system is expensive and prone to mechanical breakdown because it uses a multitude of moving parts. Other solutions have been developed using fewer moving parts.
Dual fixed-bed desiccant towers have become an industry standard as a simple solution for maintaining a constant flow of dry process air. After an adsorbent in a desiccant tower is fully regenerated, valves redirect the process air flow so that the newly regenerated tower is subsequently placed in a process cycle, while simultaneously the other tower previously in a process cycle is subsequently placed in a regeneration cycle. The towers are, therefore, always in opposite cycles (i.e. process vs. regeneration). Hence, desiccant tower switchover also assures an uninterrupted supply of dry process air with the advantage of few moving parts.
The above-mentioned equipment, however, may fail under high humidity conditions. As the humidity of the ambient air increases, the effective time period of the process cycle decreases because of a faster build-up of moisture within the surface of the desiccant. Danger arises when the tower in a process cycle becomes fully saturated before the other tower is completely regenerated.
By enlarging the size of the tower to hold more desiccant, the effective process cycle time period can be increased. This solution however is generally undesirable because the time needed to completely regenerate a fully saturated tower also increases.
Another solution is to decrease the regeneration cycle time period by more rapidly heating the saturated desiccant. U.S. Pat. No. 2,783,547 discloses dual fixed-bed desiccant towers each containing a heater coil helically wound within a central metal tube extending longitudinally along a central axis of the tower. Metal fins extend outwardly into surrounding silica gel to facilitate rapid heating of the desiccant as regeneration air is forced through the gel. Because desiccant does not have good thermal conductivity, the outwardly directed heating of the surrounding gel may be undesirably localized near the thermally conductive fins.
Similarly, U.S. Pat. No. 4,601,114 discloses a helically wound heater coil extending along the central axis of a desiccant tower. Regeneration air, however, is forced sideways from the central heater in order to heat the surrounding desiccant. Again, heating of the surrounding desiccant may tend to be undesirably localized and irregular since convection heat flow from the sideways-forced air is counter to the natural tendency of the regeneration air to rise. Hence, a more effective and even heating of the desiccant can be achieved by circulating regeneration air upwardly from below the desiccant.
In response to the above-mentioned difficulties, it is a general object of the present invention to reduce the regeneration cycle time period 25% to 30% from that of a standard desiccant tower having a single heater.
It is another object of the present invention to substantially increase the effective process cycle time period by doubling the height of a standard desiccant tower so as to ensure an uninterrupted supply of dry process air under high humidity conditions.